How do I update my iMac from High Sierra to Ventura?

Step One: Ensure that you have a current backup of your system before you attempt any upgrade.

How to Upgrade  macOS:

The macOS version you can upgrade to depends on the year your Mac was released.

Click the  (Apple) menu at the top of your screen and choose About This Mac.

Check the Compatibility of your Mac with newer versions of macOS:

➡️ macOS 10.14 Mojave is compatible with the Macs listed here

➡️ macOS 10.15 Catalina is compatible with the Macs listed here

➡️ macOS 11 Big Sur is compatible with the Macs listed here 

➡️ macOS 12 Monterey is compatible with the Macs listed here

➡️ macOS 13 Ventura is compatible with the Macs listed here

❗️ – IMPORTANT – None of your 32-bit apps will work in Catalina, Big Sur or Monterey.

32-bit app compatibility with macOS High Sierra 10.13.4 and later - Apple Support

Use the free utility Go64 to check the compatibility of your apps:

https://www.stclairsoft.com/Go64/

✅ - IF your Mac and apps are compatible, then using Safari only, download the macOS upgrade installer app:

Click >  Mojave,  Catalina,  Big Sur,  Monterey  or  Ventura.

Once downloaded, double-click the “Install macOS…” app in the Applications folder to begin the upgrade.